This invention relates generally to magnetic recording media, and more particularly to a magnetic recording disk with an antiferromagnetically-coupled (AFC) magnetic recording layer.
Conventional magnetic recording media, such as the magnetic recording disks in hard disk drives, typically use a granular ferromagnetic layer, such as a sputter-deposited cobalt-platinum (CoPt) alloy, as the recording medium. Each magnetized domain in the magnetic layer is comprised of many small magnetic grains. The transitions between magnetized domains represent the xe2x80x9cbitsxe2x80x9d of the recorded data. IBM""s U.S. Pat. Nos. 4,789,598 and 5,523,173 describe this type of conventional rigid disk.
As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (where Mr is measured in units of magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (Hc) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hc. The reason for this decrease is that the parameter Mrt/Hc is related to the ability of the recording head to resolve the magnetic bits at high density. Decreasing Mrt/Hc increases this ability. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the stored magnetic information in the layer will be more likely to decay. This decay of the magnetization has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions.
One approach to the solution of this problem is to move to a higher anisotropy material (higher Ku). However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Ms (Ms=saturation magnetization), becomes too great to be written by a conventional recording head. A similar approach is to reduce the Ms of the magnetic layer for a fixed layer thickness, which will reduce Mr since Mr is related to Ms, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer.
U.S. Pat. No. 6,280,813 describes a magnetic recording medium wherein the magnetic recording layer is at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic spacer film. In this type of magnetic media, referred to as AFC media, the magnetic moments of the two antiferromagnetically-coupled films are oriented antiparallel, with the result that the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in volume V. Therefore the thermal stability of the recording medium is not reduced. One of the ferromagnetic films is made thicker than the other, but the thicknesses are chosen so that the net moment in zero applied magnetic field is low, but nonzero. In one embodiment of the AFC medium both ferromagnetic films are sputter deposited CoPtCrB alloy films separated by a Ru spacer film that has a thickness to maximize the antiferromagnetic coupling between the two CoPtCrB films.
The use of a boron-containing alloy like CoPtCrB as the ferromagnetic film composition in an AFC medium requires the use of a special onset or nucleation layer to enhance the growth of the CoPtCrB films so that the C-axis of these films is in the plane of the films. The nucleation layer, which is typically a nonferromagnetic CoCr alloy, requires still another sputtering station in the manufacturing line. In the previously cited pending application, the lower ferromagnetic film in the AFC medium is a boron-free ferromagnetic CoCr alloy that does not require a nucleation layer between it and the Cr or Cr alloy underlayer. This ferromagnetic CoCr alloy has sufficient saturation magnetization (Ms) and grain structure to produce excellent magnetic recording performance for the AFC recording layer, while also serving as a nucleation layer to induce the in-plane C-axis growth of the upper CoPtCrB ferromagnetic film through the spacer layer.
A continuing problem in magnetic recording media is intrinsic media noise which is a significant contributor to the overall signal-to-noise ratio in the disk drive. What is needed is an AFC medium with reduced media noise.
The invention is an AFC magnetic recording medium having at least two ferromagnetic films exchange coupled together antiferromagnetically across a nonferromagnetic spacer film. In this antiferromagnetically-coupled (AFC) recording layer the magnetic moments of the two ferromagnetic films are oriented antiparallel, and thus the net remanent magnetization-thickness product (Mrt) of the AFC recording layer is the difference in the Mrt values of the two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in thermal stability of the recording medium. The lower ferromagnetic film in the AFC recording layer is a ferromagnetic CoCrFe alloy that does not require a nucleation layer between it and the Cr alloy underlayer. The AFC medium with the CoCrFe alloy as the first or lower ferromagnetic film has reduced intrinsic media noise.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.